Stents are insertable medical devices used to maintain openings for fluid flow in areas that might otherwise close, hindering flow. Stents are used to prevent restenosis after Percutaneous Transluminal Catheter Angioplasty (PTCA), presenting outward radial force against a potentially rebounding vessel wall after balloon widening. Stents are also used to hold open inflamed vessel walls that would otherwise be swollen shut, precluding flow. Stents can also be used to hold open surgically made holes for drainage.
Stents are often tubular devices for insertion into tubular vessel regions. Balloon expandable stents require mounting over a balloon, positioning, and inflation of the balloon to expand the stent radially outward. Self-expanding stents expand into place when unconstrained, without requiring assistance from a balloon. A self-expanding stent is biased so as to expand upon release from the delivery catheter.
A vessel having a stenosis may be modeled as an inwardly protruding arcuate addition of hardened material to a cylindrical vessel wall, where the stenosed region presents a somewhat rigid body attached along, and to, the elastic wall. The stenosis presents resistance to any expansion of the vessel in the region bridged by the stenosis. Stenoses vary in composition, for example, in the degree of calcification, and therefore vary in properties as well.
The arcuate geometry of many stenoses present a variation in resistance along the vessel axis to stent outward radial force. Specifically, stenosed vessel resistance is often greatest toward the middle, lessening toward the ends, with a rapid decrease at the start of healthy vessel tissue.
A conventional self-expanding stent optimally has a length greater than the length of the stenosed region to be kept open. Current stents present a substantially uniform outward radial force along their length. Currently, stents do not vary outward radial force to match stenosis geometries or resistances. A constant force stent, with sufficient force to maintain an open channel within a stenosis, has greater force than necessary in the healthy vessel portion lying past the stenosis ends. The stent ends may thus flare outward, protruding into, and possibly irritating non-stenosed tissue.
Stenosis can occur in vessel regions having asymmetric geometry lying on either side of the stenosis. One example of this is the ostium of a coronary artery, having a wide opening toward the aorta, converging into a narrower coronary artery. A conventional stent placed in the ostium would provide substantially uniform outward force over a non-uniform vessel diameter. If this force is properly matched for the narrower vessel opening, it is likely less than optimal for the wider region.
What would be desirable, and has not heretofore been provided, is a stent capable of providing sufficient force to keep a vessel open within a rebounding stenosis, while providing only necessary force against healthy, non-stenosed vessel regions. What also has not been provided is a stent providing necessary, but only necessary force along a stenosis in a vessel region having non-uniform vessel diameter on either side of the stenosis.
SUMMARY OF THE INVENTION
The present invention includes a self-expanding stent having a tubular shaped structure, where the outward radial force varies with longitudinal position along the length of the stent. In one embodiment, the force is greater in the center and lesser at both ends. Such a stent is suitable for placement in a stenosed vessel region. In another embodiment, the force is less at one end, greater at the middle, and greater still at the opposite end. Such a stent is suitable for placement in a stenosed and narrowing vessel region, including placement near a coronary ostium.
One stent has a structure formed of shape memory material. In one embodiment, the stent is constructed of a Nickel-Titanium alloy.
The stent structure in a preferred embodiment includes a helix formed of a wire having the helix turns spaced more closely together toward the center than at the ends. The helix is biased to expand in outer diameter and contract in length after having been stretched axially and released. In an alternate embodiment, the helix turns increase in spacing from one end to the opposite end. In another embodiment, interwoven or intertwined wires form the tubular structure, with the number of wires being greater per unit length toward the center than at the ends. The interwoven wires can be metallic wire. The wires can resemble spirals or helices after having been wound to the tubular stent shape. In yet another embodiment, the number of wires increase from one end to the opposite end.
One stent achieves a variation in radial force by including in the stent structure elements which intersect at junctions having more material in regions requiring more radial force and less material in regions requiring less radial force. The amount of junction material can be varied by varying the size of the junction area. In a preferred embodiment, the stent structure is formed by laser cutting a Nitinol tube, leaving a greater strut dimension in regions requiring greater outward radial force.
In yet another embodiment, the stent structure includes a series of wire springs having a "zig-zag" shape which each radially encircle a tubular section. The springs are interconnected longitudinally. The required outward radial force can be varied by varying the stent wall thickness in this and other embodiments. In one embodiment, stent regions requiring greater radial force have thicker walls than regions requiring less force.
Stents made in accordance with the present invention can provide an outward radial force more closely matching the local force requirements. In particular, the stents provide greater force only where required in a stenosis center, without providing too much force in the region of healthy tissue. The stents provide an expanded geometry more closely tailored to the requirements of a narrowing vessel region, providing greater expansion in wider regions and less expansion in narrower regions.